The -Y switch

• Suppresses the creation of Steiner points on the boundary. Internal Steiner points may still be created.
• This switch is useful when the mesh boundary must be preserved so that it conforms to some adjacent mesh.
• Be forewarned that you will probably sacrifice some of the quality of the mesh; Triangle will try, but the resulting mesh may contain triangles of poor aspect ratio. Works well if all the boundary points are closely spaced.
• Specify this switch twice (`-YY') to prevent all segment splitting, including internal boundaries. Note that this may yield a triangulation that is not conforming Delaunay.
• This switch will not disable the -s switch; if you don't want points inserted on the boundary, use -Y and don't use -s.